Nuclear magnetic resonance (NMR) testing of substances to determine the constituents therein is well known in the art. In known devices, the sample is arranged between the poles of a magnet and is enclosed by a wire coil to enable a sample to be subjected to RF electromagnetic pulses of a predetermined frequency. The resulting NMR pulse generated by the nuclei of the sample under test is detected and processed by the NMR device in a well known manner to identify the sample constituents.
NMR analysis may be performed in devices commonly known as spectrometers. These spectrometers are designed so as to have a probe, that accepts the sample to be analyzed, between poles of a magnet The RF coils and tuning circuitry associated with the probe create a field (B), that rotates the net magnetization of the nucleus. These RF coils also detect the transverse magnetization as it precesses in the X, Y plane. The RF coil pulses the sample nucleus at the Lamor frequency, so as to generate a readable signal for sample identification.
An exemplary probe that performs in accordance with that described immediately above is disclosed in commonly owned U.S. Pat. No. 5,371,464 (Rapoport), and is incorporated by reference herein. This probe, and others like it, while an improvement in the art, still had several disadvantages.
The greatest disadvantage involved temperature changes, particularly temperature increases associated with heating the magnet as a result of the strong thermal conductivity between the sample stream and the magnet itself. This is due mainly to samples that must be run through the stream at high temperatures, so as to remain liquid for analysis, and avoid gelling, solidifying or the like, if cooled. These samples typically dissipate within from the probe, that is transferred through air in the ambient environment, ultimately reaching the magnet and raising its temperature. Heat from the sample may also be transferred by radiating through the ambient environment and can be conducted through the material of the probe itself.
Since magnetic flux is proportional to magnet temperature, the magnet upon heating underwent flux changes. These changes in flux altered the homogeneity of the magnet, and thus the results obtained were inaccurate, and in some cases, worthless.
Even a small change in sample stream temperature was sufficient to cause a major change in the magnetic flux. Frequency locks, such as that disclosed in U.S. Pat. No. 5,166,620 (Panosh), were introduced into probes to counter changes in flux, by controlling the frequency of the RF coils. As for changes in magnetic homogeneity, these can only be made by shimming the magnet.
Today, when magnet control is desired in these systems, complex, highly accurate, heat exchangers are employed with these probes. These heat exchangers are placed in the path of the sample stream prior to its entry into the probe. It has been found that this solution is extremely costly and thus, difficult to implement in in-line process environments.
Additionally, the temperature conductivity between the magnet and the sample stream effects the sample itself With the sample forced to remain in the probe for the desired testing time (period) the sample itself changes as its flow has temporarily ceased during the analysis period. This temperature change can alter NMR test results.